摘要:

Near the outbound magnetopause crossing during the first encounter of Mariner 10 with the planet Mercury on March 29, 1974, large intensity, ∼6 s quasi‐periodic variations in the intensity‐time profile of the charged particle experiment's electron counting rate appeared as a series of peaks and valleys. The peaks have previously been interpreted as quasi‐periodic burst sequences of mildly relativistic (>175 keV) electrons, caused in one case by episodic ∼6‐s magnetotail substorm reconnection events and in another case by multiple encounters with a substorm energized electron population drifting around Mercury with an ∼6 s drift period. In this paper, we offer a new and fundamentally different interpretation of the Mariner 10 energetic electron, plasma electron, and magnetic field data near the outbound magnetopause at Mercury 1. We show that magnetosheath‐like boundary layer plasma was observed up to ∼360 km (∼0.15 RM) planetward of the dawn magnetopause crossing as sensed by the magnetometer. We show that observations of substorm enhanced>35 keV electron flux (that previously interpreted as>175 keV electrons) associated with the hot tenuous plasma sheet population were interleaved with ∼6 s period observations of a cold dense boundary layer plasma associated with a much lower>35 keV electron flux. We argue that the ∼6 s temporal signature is due to variation of the thickness and/or position of the boundary layer plasma population. This explanation of the ∼6‐s variations, based upon our analysis of the coincident responses of the magnetic field experiment and two independent charged particle instruments (at their highest temporal resolutions), finds a direct analogue in observations of Earth's magnetospheric boundary layer, although the time scales are signi

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06481

年代:1989

数据来源: WILEY

摘要:

The existence of a small intrinsic magnetic field at Mars is still being debated. The ion densities, measured in the Martian ionosphere above 300 km by the retarding potential analyzers on the Viking landers, are very low (Hanson et al., 1977). The ionospheric thermal pressure above 300 km is insufficient by itself to withstand the solar wind dynamic pressure, suggesting the presence of a magnetic field in the ionosphere of Mars (Hanson and Mantas, 1988). However, two types of ionospheric magnetic field are possible: (1) a weak intrinsic magnetic field, or (2) an induced magnetic field driven by the solar wind interaction with Mars. The latter situation is analogous to the Venus ionosphere during conditions of high solar wind dynamic pressure. It has been pointed out that the measured plasma scale heights and temperatures for the Martian ionosphere are very similar to those observed in the Venus ionosphere during conditions of large solar wind dynamic pressure (Luhmann et al., 1987). We present a one‐dimensional, multispecies, magnetohydrodynamic model of the Martian ionosphere. Scenarios with and without a small intrinsic field are modeled for Viking conditions (solar minimum). We find that the calculated ion density profiles are not in better agreement with the measured ones if an intrinsic magnetic field is included in the model. The results also indicate that large horizontal plasma motions must be present at high altitudes, which implies that the dynamics of the upper ionosphere of Mars is controlled by the solar win

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06506

年代:1989

数据来源: WILEY

摘要:

A model is presented to explain diffuse FUV emissions from the outer planets, specifically Uranus, in excess of those diffuse emissions that are currently explainable by scattering of sunlight and/or excitation by photoelectrons. These electroglow emissions in H Ly α and H2bands, which occur in the sunlit hemisphere slightly above the homopause, appear to require particle excitation in the 10‐ to 50‐eV range. We propose an in situ mechanism for accelerating photoelectrons (and ions) involving neutral wind dynamo generation of field‐aligned currents analogous to what occurs in the Earth's equatorialEandFregions. Sufficiently strong field‐aligned currents are found in the model calculation for Uranus to produce a potential drop of ∼100 eV or greater between theFpeak and homopause, concentrated at lower altitudes, and capable in principle of accelerating photoelectrons (and ions) to the 10‐ to 50‐eV energies required to explain the observed emissions. The fact that the excitation and ionization cross sections are larger than elastic scattering cross sections in an H2atmosphere at these energies makes in situ acceleration feasible for the production of UV on the

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06517

年代:1989

数据来源: WILEY

摘要:

Nonlinear one‐dimensional constant‐profile hydromagnetic wave solutions are obtained in finite‐temperature two‐fluid collisionless plasmas under adiabatic equation of state. The nonlinear wave solutions can be classified according to the wavelength. The long‐wavelength solutions are circularly polarized incompressible oblique Alfvén wave trains with wavelength greater than hundreds of ion inertial length. The oblique wave train solutions can explain the high degree of alignment between the local average magnetic field and the wave normal direction observed in the solar wind. The short‐wavelength solutions include rarefaction fast solitons, compression slow solitons, Alfvén solitons and rotational discontinuities, with wavelength of several tens of ion inertial length, provided that the upstream flow speed is less than the fast‐mode speed. The Alfvén solitons and rotational discontinuities are super‐Alfvénic compression waves if the upstream Alfvén‐mode speed is greater than the sound speed; otherwise, they are sub‐Alfvénic rarefaction waves. The density and magnetic field variations of these short‐wavelength waves are shown to obey the following two rules: (1) all compression waves are left‐hand polarized and all rarefaction waves are right‐hand polarized, due to the ion inertial effect, (2) the density variation and the magnetic field magnitude variation are in phase if the flow is supersonic, but out of phase if the flow is subsonic, which is a consequence of conservation of the momentum flux. The two‐fluid rotational discontinuity solution obtained in this study is highly circularly polarized, with a variable angular rotation rate. The total angle of rotation is limited to less than or equal to 180°, which is consistent with the rotational discontinuity observed in the solar wind. The upstream flow speed of the two‐fluid rotational discontinuity must deviate slightly from the Alfvén‐mode speed; the downstream flow speed is equal to the local sound speed. The formation of the two‐fluid rotational discontinuity depends critically on the dispersion effect which converts

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06523

年代:1989

数据来源: WILEY

摘要:

An overview is presented of the resistive steady state structure of intermediate MHD shocks, i.e., shocks that effect a transition from super‐alfvénic to sub‐alfvénic flow. The results are presented in terms of magnetic hodograms in which the two components of the magnetic field tangential to the shock surface are plotted against each other. By performing fixed‐point analysis in this plane, at the possible upstream and downstream states of these shocks, and by solving the one‐dimensional, steady state, resistive, nonviscous MHD equations numerically, it is found that three basic types of hodogram topology exist, describing the resistive intermediate shock structure. These topologies are characterized by the normal flow speed (in the shock frame) relative to the fast‐wave speed and the sound speed at the upstream and downstream states. Fast‐mode and slow‐mode shocks are contained within these hodograms as well. In brief summary, it is found that all intermediate shocks that have an upstream normal flow speed, νx1, less than the local small‐amplitude fast‐mode wave speed,cf1, and a downstream normal flow speed, νx2, greater than the local small‐amplitude slow‐mode wave speed,cs2, have a unique magnetic structure consisting mainly of a rotation of the tangential magnetic field, accompanied by a more or less pronounced change in field magnitude. This type of shock is called a subfast (νx1cs2) intermediate shock. A subfast strong intermediate shock has νx1

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06539

年代:1989

数据来源: WILEY

摘要:

We have analyzed the properties of ∼30 to 130‐keV/eprotons and alpha particles upstream of six quasi‐parallel interplanetary shocks that passed by the ISEE 3 spacecraft during 1978‐1979. Our measurements were performed with the University of Maryland/Max Planck Institute ULECA sensor. We deduced the energetic particle diffusion coefficients, κ∥, by calculating the difference between the bulk flow speed of the particles and their estimated scattering center speeds in the spacecraft frame and relating this to the particle spatial intensity gradients upstream. This calculation yielded diffusion coefficients for each 2‐min interval in the last ∼15 min before shock passage, when the particle intensities were high. Comparing these diffusion coefficients in detail with the theory of Lee (1983), we find that upstream of the shock theA/Qdependence of κ∥for alphas and protons agrees well with the theory. Upstream of the shock, κ∥often increases with distance, as predicted by Lee's theory, although the details of this increase do not agree with the theory. At the shock front itself we find excellent agreement between theory and observation for (1) the velocity dependence of κ∥, and, (2) the dependence of κ∥on the magnitude of the pr

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06552

年代:1989

数据来源: WILEY

摘要:

Solar wind thermal electrons are normally nearly isotropic, with a temperature ratio,T∥/T⊥, of 1.2 or less. This study presents ISEE 3 observations of unusually anisotropic (T∥/T⊥= 1.5 to 4) electron distributions in the solar wind near 1 AU. The highly anisotropic events generally share the following characteristics: (1) they have unusually low density, and (2) they are located in the rarefaction regions on the trailing edges of high‐speed streams. Correlation of the electron temperature ratio with other solar wind parameters over a full range ofT∥/T⊥shows a distinct negative correlation with electron density and reveals that the anisotropies are caused by highT∥rather than by lowT⊥. A simple numerical model illustrates that these effects can be described by the competing processes of solar wind expansion and isotropization via Coulomb self‐collisions. However, disagreement between model predictions and observations suggests the need for consideration of other mechanisms, including wave‐particle interactions, for repartitioning of electron energy between the parallel and perpendicular components. The periods of extreme electron anisotropy tend to be coincident with intervals of double ion beams, suggesting similar causal mechanisms for the two phenomena. We conclude that certain aspects of the thermal anisotropies can be explained in a simple manner but that complete understanding will require analysis of additional factors such as collisions in nonthermal distributions and the radial evolution of solar wind structures. In the appendix, analysis of the alignment of the major axes of the electron distributions with the observed magnetic field directions demonstrates that calculation ofT∥andT⊥self‐consistently from the electro

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06563

年代:1989

数据来源: WILEY

摘要:

The resonant absorption of Alfvén waves in the plasma sheet boundary layer (PSBL) provides a significant source of energy for the heating of plasma sheet particles. We show that the energy absorbed is a function of the central plasma sheet (CPS) temperature. The heating curve when coupled with convective transport yields an equation of state for the steady state plasma sheet whose solution has the form of a mathematical catastrophe. We have previously suggested that this catastrophe is associated with the abrupt increase of energy dissipation during the onset of the substorm expansion phase. In this paper our master equation is generalized to include convection velocityVzself‐consistent with pressure anisotropy, and to retain the dynamics describing the transition across the thermal catastrophe. The dynamic terms allow evaluation of the time scale for the catastrophe to occur. The evolution of the plasma sheet through the growth phase to onset is traced in the quasi‐static limit, assuming that the system passes slowly through a succession of equilibrium states described by the stationary limit of the master equation. The state variable for the plasma sheet is the temperatureT; the control variables are the evolving lobe fieldBℓand the incident power flux, represented byW∼b², wherebis the amplitude of the driving waves at the surface of the PSBL. Other parameters are reduced to initial conditions and then scale self‐similarly with the evolvingBℓ. For a physically reasonable range ofW, catastrophe occurs asBℓincreases above a critical value, and in a relatively narrow range of local time, nominally 2200

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06581

年代:1989

数据来源: WILEY

摘要:

Using four months of tail data obtained by the three‐dimensional plasma instrument on board the AMPTE/IRM satellite in 1986, we have done a statistical survey on the behavior of ion and electron moments in the central plasma sheet. Almost 80,000 spin averages of plasma density, ion bulk velocity, ion and electron temperature, and plasma β were analyzed with respect to differences between their values in the inner and outer central plasma sheet as well as their dependence on magnetic activity. The ion temperature increases with increasing magnetic activity while the ion density decreases during disturbed intervals, except in the neutral sheet neighborhood at smaller radial distances. The ion and electron temperatures in the central plasma sheet are highly correlated, withTi/Tebeing constant over a wide range of temperatures and about twice as large as in the distant tail. The average ion flow speeds in the central plasma sheet are below 100 km/s and nearly identical to those found in the plasma sheet boundary layer, although the distribution functions usually are quite different. High‐speed flows do occur, but in bursts of most often less than 1 min duration with intermittent intervals of nearly stagnant plasma. The distribution of flow directions strongly favors sunward flow for velocities above 300 km/s, indicating that a near‐earth neutral line is rarely, if ever, located inside ofXGSM=

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06597

年代:1989

数据来源: WILEY

摘要:

Magnetic field and particle flux observations on board ATS 6 at synchronous altitude are used to examine the dawn‐dusk asymmetry of characteristics of Pc 4–5 waves and associated particle flux modulation. Most waves at synchronous orbit having ground correlations are polarized in the azimuthal direction (A class) and are usually detected in the dawn sector. Waves with a radially oriented polarization ellipse (R‐class) are almost never observed near the subsatellite point on the ground, except for the regular pulsations known as giant pulsation Pg, observed in the early morning. R class Pc 4 waves occur at all local times and have an occurrence peak in the afternoon. It is found that A class Pc 4–5 waves are observed during moderately disturbed conditions, associated with a solar wind velocity of about 600 km/s or more. R class Pc 4 waves tend to predominate in the afternoon sector under quiet magnetic condition. The modulation of proton fluxes in the energy range 25–500 keV depends on the mode of the associated magnetic waves. The phase relationship between magnetic field and proton flux oscillations is simpler for the A‐class waves as compared with the R class waves. The oscillation of the azimuthal component is out of phase with proton flux oscillations in the energy below about 120 keV. Proton flux modulation is larger at pitch angle near 90° than it is at smaller angles. In contrast, the phase relationship is not so simple for R class Pc 4 waves which are the dominant waves in the afternoon sector. For these waves the phase depends on both energy and pitch angle. Two thirds of the R class Pc 4 events examined show a large modulation at small pitch angle of 30°–40°. A large‐amplitude pulsation of R class, storm‐time Pc 5, observed in the afternoon is associated with proton flux oscillations around 90° pitch angle. For these Pc 5 waves the proton flux variations are out of phase with the compressional component of magnetic field, and their phase is almost independent of energy. Implications of the observed dawn‐dusk asymmetry in characteristics of the azimuthal and radial wave modes and their associated proton flux mo

ISSN:0148-0227    

DOI:10.1029/JA094iA06p06607

年代:1989

数据来源: WILEY